PROJECT SUMMARY Patients with chronic kidney disease (CKD) experience a wide-range of bone and mineral metabolism abnormalities, including bone mass loss and excess production of fibroblast growth factor 23 (FGF23) by osteocytes. Serum FGF23 levels rise early in the course of CKD, and high levels are independently associated with greater risks of CKD progression, cardiovascular disease and death. These findings have stimulated interest in designing therapies to reduce bone abnormalities and lower FGF23 levels, but the approach is limited by poor understanding of the molecular mechanisms at play. In preliminary data of this proposal, we show that the nuclear receptor, hepatocyte nuclear factor 4 alpha (HNF4?) is mechanistically linked to bone specific pathways controlling FGF23 production in animal models of FGF23 excess. We also show that FGF23 promoter contains a predicted HNF4 response element (HNF4-RE) and that Hnf4? is expressed in osteoblasts and osteocytes. In new preliminary data, we show that HNF4? stimulates osteoblast proliferation and that osteoblast specific deletion of Hnf4? leads to FGF23 excess in osteoblasts and mice, and to low bone mass in animals. We also show that Hnf4? expression is repressed by 98% in bones from an established model of progressive CKD, the Col4a3KO mouse. These observations support an important new role for HNF4?. In this proposal, we will test the hypothesis that HNF4? deficiency in CKD contributes to excess FGF23 and bone abnormalities. In Aim1 we will establish the role of HNF4? in bone, by assessing in vitro the proliferation, activity and differentiation of osteoblasts that overexpress or lack HNF4?. We will characterize the bone HNF4? transcriptome by RNA sequencing and complete the bone phenotype analysis of mice carrying the specific deletion of Hnf4? in osteoblasts/osteocytes (Hnf4?Oc-cKO). To investigate whether HNF4? mediates the osteogenic response, we will expose WT and Hnf4?Oc-cKO mice to catabolic and anabolic PTH challenges. In Aim2 we will investigate HNF4? as a molecular suppressor of FGF23 production in bone, by testing whether HNF4? directly binds to HNF4-RE on FGF23 promoter to inhibit FGF23 transcription. We will also determine FGF23 response to HNF4? inhibition in mice with low FGF23 production. Finally, in Aim3 we will test the hypothesis that in CKD, decreased HNF4? activation stimulates FGF23 transcription. We will determine when HNF4? expression decreases during CKD progression and whether HNF4? responds to phosphate. We will demonstrate the therapeutic potential of targeting HNF4? pathways by crossing Col4a3KO to newly generated mice containing a bone targeted overexpression of Hnf4? (HNF4?Oc-cTg) and by assessing correction of FGF23 levels and bone mass in Col4a3KO/Hnf4?Oc-cTg compound mice. The project will contribute to new insights into the molecular regulation of bone and FGF23 in health and in CKD, and support our ultimate goal of developing novel therapeutic approaches to improve outcomes in CKD.